oxidation of hydrocarbons abstract -...

CHAPTER 4

OXIDATION OF HYDROCARBONS

Abstract

The development of heterogeneous catalysts for selective oxidation of

hydrocarbons is a current challenge and has been studied extensively in recent

years. Due to environmental and economic concerns, the development of

highly efficient catalytic processes, which minimize the formation of side

products and residues, is quite desirable. Oxidations using environmentally

friendly oxidants such as molecular oxygen, hydrogen peroxide and t-butyl

hydroperoxide are more desirable these days. Catalytic oxidation offers the

advantage that volatile organic compounds can be removed from aerial

effluents to very low levels. In this chapter, the liquid-phase oxidation

reactions offour hydrocarbons over chromite spinel catalysts are analyzed in

detail. They are the oxidations of benzyl alcohol, styrene, cyclohexane, and

ethylbenzene. The influence of various reaction parameters was investigated

thoroughly. Possible reaction pathways involved in each oxidation were

proposed.

Chapter 4

SECTION: A

4.1. OXIDATION OF BENZYL ALCOHOL

4.1.1. Introduction

Catalytic conversion of primary alcohols to aldehydes is essential for the

preparation of fragrances and food additives as well as many organic intermediates 1.

Traditional methods for the synthesis of aldehydes generally involve the use of

stoichiometric amounts of inorganic oxidants such as, CrVl, and generate large

quantities of waste. Hence the development of effective and environmentally benign

heterogeneously catalyzed oxidation of alcohols is an important challenge. Metal

catalyzed reactions using molecular oxygen instead of mineral oxidizing agents, are

particularly attractive for environmental reasons. Pd and Pt metals supported on

alumina are among the widely used metal catalysts in the selective oxidation of

alcohols. Such systems, however, tend to deactivate quickly due to the strongly

adsorbed products or by-products formed during the reaction2.

Metal oxides were found to be effective in the catalytic oxidation of benzyl

alcohol. Stuchinskaya and Kozhevnikoy3 have reported heterogeneous oxidation of

benzyl alcohol to benzaldehyde by O2 in liquid phase at lOO°C and ambient pressure

using hydrous binary PdlI -metal oxides as catalysts. Modification of Pd (II) oxides

with transition metal cations generally improved the catalytic activity and selectivity

to aldehyde, Co (III) and Fe (Ill) being the most effective promoters. The oxidation of

alcohols on Pd-M oxide catalyst was accompanied by transfer hydrogenation and

decarbonylation side reactions, which were similar to the oxidation on the Pd metal.

This indicated that the oxidation of alcohol on Pd-M oxide catalysts occurred via a

dehydrogenation mechanism with hydrogen being present on the catalyst surface.

96

Oxidation of Hydrocarbons

Nano sized NiOz powder was applied as a catalyst for benzyl alcohol oxidation by Ji

et a1.4• Liotta et a1. 5 reported the structural and surface characterization of chromia on

silica and zirconia oxides and its catalytic performance towards benzyl alcohol

oxidation. A correlation was found between the benzyl alcohol conversion and the

amount of chromia reduced obtained from TPR of used samples.

MCM-41 supported hydroxo-bridged dicupric-phenanthroline complex were

found to be efficient catalyst for the oxidation of benzyl alcohol with TBHp6.

Ganeshan and Viswanathan synthesized ~-oxo bridged dinuclear iron 1.10-

phenanthroline complex encapsulated in MCM -41 and compared benzyl alcohol

oxidation in both neat and encapsulated complexes7. In neat complex. Fe-O-Fe bridge

was cleaved during oxidation while in encapsulated system, it was stable. Farukawa et

al. studied gas phase catalytic oxidation of benzyl alcohol over various zeolites

catalysts. They have reported the effect of alkali metal doping to supported La/ZSM-5

catalysts on the catalytic activity of gas-phase oxidation of benzyl alcoholB and a

promotion scheme for the alkali metal added to the La/NaZSM-5 catalyst was

performed. Also the effect of alkali promotion on Cu-Na-ZSM-5 catalysts9, Co (II)

NaY lO zeolites and copper exchanged V-type zeolites ll for benzyl alcohol oxidation

were reported.

This section presents an exhaustive investigation on the liquid phase oxidation

of benzyl alcohol with H20 2 over the prepared catalytic systems. The general scheme

of the reaction represented in scheme 4.1.1. The main reaction product is

benzaldehyde, which is then oxidized to benzoic acid. All the catalytic systems

showed considerable activity towards the oxidation with high selectivity towards

benzaldehyde.

97

Chapter 4

[0] [0]

Benzyl alcohol Benzaldehyde Benzoic acid

Scheme 4.1.1: Reaction scheme of benzyl alcohol oxidation

H20 2 has many advantages as oxidant because water is the only expected side

product and it is easy to be dealt with after reactions. Meanwhile. dilute aqueous H20 2

(concentration less than 60%) solution is safe. non-toxic. and low cost. However.

aqueous HZ0 2 is a moderate inorganic oxidant. and it does not form a homogeneous

solution with most organics. The other problem of aqueous HZ0 2 as oxidant rises from

its poor stability because it is liable to decompose while heated or in the presence of

many metal ions. non-metal ions and finely ground particles. Therefore there is a

competition between the decomposition reaction and oxidation reaction. The above

disadvantages of aqueous H20 2 limit its application in organic oxidation reactions.

Accordingly. the key problem of relevant research is to look for efficient catalyst that

can activate but not decompose HZ0 2•

4.1.2 Influence of Reaction Conditions

Influence of reaction conditions is essential for a chemical reaction to occur

with high percentage conversion and selectivity for products. The influence of

different reaction parameters was analyzed in order to maximize the product yield and

selectivity. Effect of reaction conditions for benzyl alcohol oxidation with HZ0 2 was

initially assayed in non-optimized conditions with CCoCr-2 as the catalyst.

4.1.2.1 Effect of Time

98


The influence of reaction time on benzyl alcohol oxidation is illustrated in

figure 4.1.1.

.... Benzyl alcoml --.- Benzoic add --0-- Eerrzaldeh)tie

60 .,-----t"..--------------..,. 60

~50

! 640 .;:;; ... ... > c:: 830

55

20 +-----.----.,...-----,----+ 40

3 6 9 12

Reaction conditions: Catalyst-CCoCr-2, Temperature-80°C, Benzyl alcohol: H20 2 =1:4, Catalyst weightO.lg, Solvent-1 Oml Acetonitrile

Figure 4.1.1: Effect of time on benzyl alcohol oxidation

After a 6 h run, a benzyl alcohol conversion of 32% was attained and this

value remained steady throughout. Selectivity towards benzaldehyde decreased

continuously while the reverse occurred for benzoic acid. This may be because

consecutive oxidation of product benzaldehyde was favored with increasing time. A

time on stream of 6 h was selected for further studies.

4.1.2.2 Effect of Temperature

The effect of temperature on benzyl alcohol oxidation was studied in a

temperature range of 2S-80c C while all other parameters were kept constant. Results

are presented in figure 4.1.2.

99

Chapter 4

..... Benzyl alcohol ....... Benzoic acid ~ Benzaldehyde

50 r---=::&====~------180

~ 40

! c .~ 30

t ?: 8 20

10 +---...----.----.----r----+ 25 50 60 70 80

Temperature('C)

70

60 l 0 50 .;;: ·B 41

40 jl

30

20

Reaction conditions: Catalyst -CCoCr-2. Time-6 h. Benzyl alcohol: H20 2 '=1: 4. Catalyst weight-O.lg. Solvent -1 amI Acetonitrile

Figure 4.1.2: Effect of temperature on benzyl alcohol oxidation

As temperature increased. benzyl alcohol conversion also increased till 70°C

and later a decrease was observed. This decrease may be attributed to enhanced

decomposition of H20 2• which was facilitated at higher temperature. Benzaldehyde

selectivity increased up to 60°C. At higher temperatures. benzoic acid selectivity

increased at the expense of benzaldehyde selectivity. This may be related to the

activation energies for the reaction: higher temperature favor reactions with higher

activation energy. Higher temperature also favors the successive steps in consecutive

reactions. A temperature of 60°C was selected for further studies so that higher

benzaldehyde selectivity was obtained.

4.1.2.3 Effect of Reactant Mole Ratio

Figure 4.1.3 summarizes the influence of molar ratio of benzyl alcohol to

H20 2 in the oxidation over CCoCr-2 catalyst.

100


_____ Benzyl alcold ~ Benzoic add -<)- futzaJ.dehyde

40.,....---------------.----.00

35

* -'30 ~ c:

.S: 25 ~ ., ~ 20 o

U IS

80

70

6O~ ~

50 :e ti

40..2:: ., Cl)

30

20

10+--------,------,------,---,------+10 1:01 1:02 1:03

MIle ratio

1:04 1:05

Reaction conditions: Catalyst-CCoCr-2. Time-6 h. Ternperature-70°C. Catalyst weight-O.1g. Solvent-l 0 ml Acetonitrile

Figure 4.13: Effect of mole ratio on benzyl alcohol oxidation

Benzyl alcohol conversion increased up to a mole ratio of 1:4 after which a

decrease was observed. This may be due to an enhancement of self-decomposition of

oxidant at higher concentration. Formation of benzoic acid was promoted with

increasing amount of oxidant. The presence of excess oxidant favored further

oxidation of initially favored product. benzaldehyde. A mole ratio of 1:2 was selected.

4.1.2.4 Effect of Catalyst Weight

The activities for benzyl alcohol oxidation over CCoCr-2 with different

catalyst amount are presented in figure 4.1.4.

Benzyl alcohol conversion remained steady initially and later showed a

moderate decrease. This was because a large amount of the catalyst hastened the

decomposition of HzOz. An amount of O.lg of catalyst was selected.

101

Chapter 4

____ Benzyl alcohol -.- Benzoic acid --0- Benzaldehyde

0.05 0.1 0.15 0.2

AmOW1! of catalyst (g)

Reaction conditions: Catalyst-CCoCr-2. Time-6 h. Temperature-70°C. Mole ratio- 1:4. Solvent-lOml Acetonitrile

Figure 4.1.4: Effect of catalyst weight on benzyl alcohol oxidation

4.1.2.5 Effect of Solvent

To investigate the role of solvent on the oxidation of benzyl alcohol. reaction

was carried out in methanol. dichloromethane. benzene and acetonitrile. The influence

of these solvents on benzyl alcohol conversion is shown in figure 4.1.5.

The enhanced activity and moderate selectivity of the catalysts in acetonitrile

could be explained on the basis of polarity of these solvents. In organic solvent the

reaction is initiated by electron transfer at the interface leading to the radical cation of

the substrate and super oxide anion while in the aqueous solution, the actual active

species are assumed to be hydroxyl radicals formed by oxidation of solvent.

Acetonitrile is an aprotic solvent. The activity of the catalysts was found to increase

with the solvent polarity and acetonitrile having more polar nature always enhanced

102


the activity. In this solvent, the phase separation between the aromatic substrate and

the aqueous oxidant was greatly decreased thereby allows an easy transport of the

active oxygen species for the oxidation.

___ Benzy I alcohol -.- Benzoic acid --<::- Benzaldehyde

40 90

80

70 * 60 ~ .., 50 :s 40 Z

.!1 .. 30 (J"J

.~ 3S

.1 30

= 25 '6 .. 20 > c

'" 20

U 15

10 IQ

A B C D

Solvent A- Methanol. B-Dichloromethane. C-Benzene.

D-Acetonitrile

Reaction conditions: Catalyst-CCoCr-2, Time-6 h. Temperature-70°C. Mole ratio- 1:4. Catalyst weight-0.1 g. Solvent volume-l Oml

Figure 4.1.5:Effect of solvents on benzyl alcohol oxidation

4.1.3 Benzyl Alcohol Oxidation over the Prepared Catalysts

The oxidation of benzyl alcohol reaction was carried out over all the prepared

catalysts under the selected reaction conditions (table 4.1.1) with the aim to produce

benzaldehyde more selectively.

Table 4.1.1: Optimized reaction conditions for benzyl alcohol oxidation

Reaction Parameters

Temperature

Time

Benzyl alcohol: HZ0 2 ratio

Catalyst weight

Solvent

Selected condition

60°C

6h

1:2

0.1 g

Acetonitrile 10 ml

103

Chapter 4

The activity for benzyl alcohol oxidation over the five series of chromite

spinel catalysts is presented in table 4.1.2.

Table 4.1.2: Catalytic activity of spine Is in benzyl alcohol oxidation

Catalyst Benzyl alcohol Selectivity (%)

conversion (wt %) Benzaldehyde Benzoic acid

CCr 20.3 84.2 15.8

CFCr-l 24.0 78.7 21.3

CFCr-2 24.3 90.7 9.3

CFCr-3 30.2 73.6 26.4

CF 21.2 73.9 26.1

CMCr-l 26.5 72.4 27.6

CMCr-2 27.2 68.1 31.9

CMCr-3 26.8 61.5 38.5

MCr 15.9 58.9 41.0

CCoCr-1 26.6 80.6 19.4

CCoCr-2 27.3 76.4 23.6

CCoCr-3 24.1 67.1 32.9

CoCr 27.1 65.9 34.1

CNCr-1 21.9 86.7 13.3

CNCr-2 26.2 60.5 39.5

CNCr-3 26.1 66.6 33.4

NCr 15.9 73.9 26.1

CZCr-1 21.6 85.6 14.4

CZCr-2 27.4 67.1 32.9

CZCr-3 19.6 80.2 19.8

ZCr 15.1 73.1 26.9

104


4.1.4 Regeneration and Stability

To study the stability of the catalysts, recycling experiments with regenerated

catalysts were carried out. The procedure adopted was as follows. After 6 h reaction,

the catalyst was recovered by hot filtration, washed several times with acetone, dried

at BO°C overnight and calcined for 8 h at 650°C. The recovered catalysts were reused

for benzyl alcohol oxidation under the same reaction conditions. The result obtained

with regenerated catalysts is presented in table 4.1.3.

Table 4.1.3: Activity of regenerated catalysts

Catalyst Cycle Benzyl alcohol Product selectivity (%)

conversion (wt %) Benzaldehyde Benzoic acid

1 26.2 60.5 39.5

2 25.9 65.3 34.7

CNCr-2 3 22.9 76.6 23.4

4 19.5 74.8 25.2

Benzyl alcohol conversion remained almost constant for the first two cycles

and showed a decrease for the third and fourth cycle. There was a significant variation

in benzaldehyde selectivity up to the third cycle, after which it remained nearly

constant.

4.1.5 Discussions

The oxidation of benzyl alcohol to benzaldehyde was carried out over copper

chromite and transition metal substituted copper chromite spinel catalysts. In almost

all catalysts, above 20% benzyl alcohol conversion was achieved along with more

than 60% selectivity towards benzaldehyde. It was observed that the product

benzaldehyde had a tendency to oxidize to benzoic acid under the same reaction

105

Chapter 4

conditions. Another observation was that simple chromites were less active than

mixed chromite systems.

Iron substitution lead to an enhancement in the catalytic activity of copper

chromite. On iron substitution, benzyl alcohol conversion increased whereas copper

ferrite showed less activity than the solid solutions. In spinel catalysts, octahedral

metal ions are exposed to the surface and are more active in catalytic reactions. In

solid solutions containing both chromium and iron in octahedral position are exposed

to surface and they showed higher activity. Among these systems, CFCr-3 showed

maximum benzyl alcohol conversion and moderate selectivity to benzaldehyde.

Substitution of copper by manganese in copper chromite spinel improved the

catalytic activity. Among manganese containing solid solutions, the catalyst with

composition CUo.sMno.SCr204 showed maximum conversion (copper and manganese

are in 1: 1 ratio in the tetrahedral position). Though the catalytic activities of spinels

depend mainly on the octahedral metal ion, tetrahedral metal ions may have some

influence. Manganese chromite exhibited the lowest activity in this series of spinel

catalysts.

Cobalt substitution increased the catalytic activity of copper chromite spinet.

Benzaldehyde selectivity was lowered upon cobalt substitution. This may be because

cobalt enhanced the successive oxidation of benzaldehyde.

Nickel substituted copper chromites were more active than the parent spinet.

A higher conversion was achieved with the catalyst Cuo.sNio.sCr204' Nickel chromite

showed the least activity. There was a significant improvement in benzaldehyde

selectivity also. Similarly, zinc substitution enhanced the catalytic activity of copper

chromite. A greater alcohol conversion was achieved with CUO.5ZnO.SCrZ04' Zinc

chromite showed the least activity.

106


4.1.6. Mechanism of the reaction

Different mechanisms have been suggested by various authors for the

oxidation of benzyl alcohol with hydrogen peroxide. Zbigniew has reported the

mechanism and kinetics of epoxidation of allyl alcohol by H20 2 with tungstic acid as a

catalyst l2. A peroxo complex formed from the tungstic acid and HzOz acts as an

oxidizing agent. Venturello and Ricci have proposed that the oxidative cleavage of 1,

2-diols to carboxylic acids by HzOz in the presence of tungstate and phosphate (or

arsenate) ions proceeds via formation of peroxo intermediate!3. Jacobson et a1. 14 have

proposed a similar mechanism for the oxidation of monohydric alcohols catalyzed by

oxodiperoxo tungstate (VI).

A plaUSible mechanism for the oxidation of benzyl alcohol with H20 Z is

described below. At first, a peroxo complex is formed by the reaction between HzOz

and the catalyst. In the second stage, the peroxo complex and benzyl alcohol react to

give an intermediate. This intermediate, on loss of water molecule. gives

benzaldehyde and the regenerated catalyst.

4.1.1. Conclusions

The summary of the results of the various studies is presented below:

.:. Copper chromite and transition metal substituted copper chromites effectively

catalyzed the oxidation of benzyl alcohol with hydrogen peroxide .

• :. The reaction always gave benzaldehyde as the oxidation product and benzoic

acid was formed by the oxidation of benzaldehyde .

• :. Reaction variables such as reaction time, temperature of the reaction. benzyl

alcohol to hydrogen peroxide ratio, catalyst weight and solvent used are the

indispensable factors influenCing the catalytic activity of the systems.

107

Chapter 4

.:. Regeneration and stability of the catalysts were studied and the results proved

that they are stable up to four reaction cycles .

• :. Mixed solid solutions exhibited improved activity.

************

108


SECTION: B

4.2. OXIDATION OF STYRENE

4.2.1. Introduction

The oxidative conversion of olefins to aldehydes and ketones is important in

chemical industry. The current practices can be divided into three categories: (i) the

cleavage of C=C bond over materials such as osmium tetroxide and ruthenium

tetroxide in stoichiometric amountl-3

, (ii) the ozonolysis of olefins to ozonides and the

subsequent conversion to aldehydes or ketones in reductive workup conditions3.4 and

(Hi) the oxidation of olefins by hydrogen peroxides.6.

Styrene oxidation is of considerable commercial and academic interest for the

synthesis of important products such as benzaldehyde, styrene oxide and phenyl

acetaldehyde. Two major reactions take place during styrene oxidation depending on

the nature of the catalyst and the reaction conditions. They are the oxidative C=C

cleavage into benzaldehyde and epoxidation followed by isomerisation into phenyl

acetaldehyde. The reaction pathways involved in the styrene oxidation is shown in

scheme 4.2.1.

A number of workers investigated styrene oxidation on various catalysts. In

spinel catalysts, the major reaction taking place is the oxidative C=C cleavage into

benzaldehyde. Ma et a1.7 studied styrene oxidation over nanosized spinel type

MgxFe3x04 complex oxides prepared by co precipitation and citrate gel method. Their

results predicted that catalysts obtained by citrate gel method are more active for

oxidation of styrene with H20 Z as oxidant. due to their higher dispersity and smaller

109

Chapter 4

particle size. The presence of highly dispersed a.~Fe203 in the spinet matrix was

probably the cause for the increased activity of the non-stoichiometric catalysts.

Manorama and co-workers reported styrene oxidation with H20 2 over Ni. Fe and Zn

ferrites and a plausible mechanism involved in the catalytic reaction was proposed8.

Their observations showed that, among all complex ferrites, Fe304, synthesized at

around pH 7 was found to be most effective for styrene oxidation to benzaldehyde.

This may be due to a large number of oxygen vacancies on the surface.

OH_2 ----i .. ~ Styrene \,

\ '\

\\,

Styrene oxide

I o

Benzaldehyde

..

Phenyl acetaldehyde

Scheme: 4.2.1: Reaction scheme of styrene oxidation

Styrene oxidation by manganese schiff base complexes in zeolite structures

was studied by Silva et a1. 9 They predicted that both neat and encapsulated Mn (Ill)

complexes were active in oxidation and the catalytic activity pattern did not change

upon encapsulation. The major product was benzaldehyde followed by styrene oxide.

The effect of catalysts such as iron porphyrins lO, 'salen-type' Mn (HI) catalysts

110


derived from D-glucosell and metalloporphyrins l2 in the oxidation of styrene with

various oxidants have been investigated and discussed by some workers.

Functionalized mesoporous silica was found to be better catalysts in styrene

oxidation. Luo and Un synthesized Co (II) salen functionalized MCM -41 type hybrid

mesoporous silica and they are applied as catalysts for styrene oxidation with HzOP.

The silica framework kept the active sites dispersed resulting in the formation of

active heterogenized catalysts for the liquid phase oxidation of styrene with excellent

stability against leaching. Transition metal incorporated (Mn, V and Cr) MCM -48

materials were found to be very active in oxidation of styrene l4 and their activity

depend on the nature of the transition metal used. Titanium substituted SBA-lS

mesoporous molecular sieves lS.16

, mesoporous nickel silicate membranes on porous

alumina supports l7 and titanium silicalite zeolites18 were found to be act as catalysts in

styrene oxidation.

In this section. a detailed investigation of the prepared spinel catalysts for

styrene oxidation with TBHP as oxidant has been carried out. In addition, the

influence of reaction parameters such as reaction time, temperature, styrene: TBHP

mole ratio, catalyst weight and effect of solvent have also been discussed.

4.2.2. Influence of Reaction Conditions

Influence of reaction conditions is essential for a chemical reaction to occur

with high percentage conversion and selectivity for products. The influence of

different reaction parameters was analyzed in order to maximize the product yield and

selectivity. Effect of reaction conditions for styrene oxidation with TBHP was initially

assayed in non-optimized conditions with CCoCr-2 as the catalyst.

111

Chapter 4

4.2.2.1. Effect of Time

In heterogeneous catalysis the formation and selectivity of products always

depends upon the reaction time. Effect of time on styrene oxidation is shown in figure

4.2.1.

90 ___ Styrene -ll- Benzaldehyde 80

75

70 t >.

65 :'E tz

60 ~ tI'J

55

40+---r--.~~--~---r--~--+50

3 4 6 8 ID 12 24

Time (h)

Reaction conditions: Catalyst -CCoCr-2, Temperature-70°e, Styrene: TBHP=1:2, Catalyst weight-O.lg, Solvent-lOml Acetonitrile

Figure: 4.2.1: Effect of time on styrene oxidation

Styrene conversion increased with reaction time. Benzaldehyde selectivity

increased steadily up to 8 h and later declined. As time progressed, formation of other

side products increased, resulting in reduced benzaldehyde selectivity. A time on

stream of 8 h was selected in order to get maximum selectivity to benzaldehyde.

4.2.2.2. Effect of Temperature

The dependence of reaction temperature on benzaldehyde production was

studied by varying the temperature between 50 and 80°C while other parameters were

kept constant. Results are presented in figure 4.2.2.

112


75 ___ Styrene -(;r Benzaldehyde 75

70

'* 65 -;

~ 60 '" ~

r/)

55

35+-----~----~------~----+ 50

50 60 70 80

Temperature ('C)

Reaction conditions: Catalyst-CCoCr-2. Time-8 h. Styrene: TBHP= 1:2. Catalyst weight-O.lg, Solvent-10ml Acetonitrile

Figure: 4.2.2: Effect of temperature on styrene oxidation

When reaction temperature was raised, styrene conversion improved

dramatically up to 70°C after which a decrease was observed. Higher temperature

favored C=C bond cleavage which explained the increase in conversion.

Benzaldehyde selectivity showed a similar behavior but with a moderate increase

only. Above 70°C. self-decomposition of TBHP proceeded faster and it did not

participate effectively in the oxidation process. Similarly. formation of styrene

polymers was observed and hence a decreased selectivity towards benzaldehyde

occurred. A temperature of 70°C was selected for further studies.

4.2.2.3. Effect of Reactant Mole Ratio

The effect of the styrene to TBHP mole ratio on the oxidation was

investigated and the results are shown in figure 4.2.3.

Styrene conversion increased dramatically with increased concentration of

TBHP and the value touched 100% at a mole ratio of 1:5. Selectivity to benzaldehyde

showed a marginal variation only. Moderate conversion and maximum selectivity was

113

Chapter 4

achieved with a mole ratio of 1: 2 and this concentration was selected for the further

studies of the reaction.

100 -... StyTene -t:r- Benzaldehyde 75

90

* 80 70

1 70 I: 0

~ 60 .. .. I: SO 0

U 55 40

30 +-----~----~----~----__+50

I:O! 1:02 1:03 [:05

Styrene: TB HP ratio

Reaction conditions: Catalyst-CCoCr-2. Temperature-70°C, Time-8 h. Catalyst weight-O.lg, Solvent-lOml Acetonitrile

Figure: 4.2.3. Effect of mole ratio on styrene oxidation

4.2.2.4. Effect of Catalyst weight

The dependence of the amount of the catalyst on the production of

benzaldehyde is presented in figure 4.2.4.

When the amount of catalyst was increased to O.1g. styrene conversion

increased Significantly. Later it showed a marginal decrease with higher catalyst

amounts. Selectivity towards benzaldehyde was nearly constant initially and showed a

moderate decrease at higher amounts of catalyst. The dependence of product

formation on catalyst concentration suggested that the reaction proceeded in a purely

heterogeneous fashion. The catalyst amount selected was O.lg.

114

75

~ 70

* 1. 65 .§ E 60 > a u 55


........ Styrene -tr- Benzaldehyde 70

68

62

50 +----.------,---r----t 60 0.05 0.1 0.15 0.2

Amount of catalyst(g)

Reaction conditions: Catalyst-CCoCr-2, Temperature-70°C, Time-8h, Styrene: TBHP ratio-I: 2, Solvent-IOml Acetonitrile

Figure: 4.2.4. Effect of catalyst weight on styrene oxidation

4.2.2.5. Effect of Solvent

In order to investigate the role of solvent, the oxidation of styrene was carried

out in propan-2-01, benzene, methanol and acetonitrile. The influence of these solvents

on styrene conversion is shown in figure 4.2.5.

The reaction media had a strong influence on the activity of the catalysts.

Acetonitrile was found to be the best solvent in terms of both conversion and

selectivity. The enhanced activity and selectivity of the catalysts in acetonitrile could

be explained on the basis of polarity of these solvents. In organic solvent, the reaction

was initiated by electron transfer at the interface leading to the radical cation of the

substrate and super oxide anion while in the aqueous solution, the actual active

species were assumed to be hydroxyl radicals formed by oxidation of solvent. The

activity of the catalysts was found to increase with the solvent polarity. Acetonitrile,

an aprotic solvent had more polarity which explained the enhanced activity. In this

solvent. the phase separation between the aromatic substrate and the aqueous oxidant

115

Chapter 4

was greatly decreased thereby an easy transport of the active oxygen species took

place.

70

60

*' ! 50

.~ 40 .. ., ~ 30 =>

U 20

___ Styrene 1:r Benzaldehyde

10+-----~----~------~----+

A B c D Solvent

A-Propan·2-oL B·Benzene. C-Methanol. D·Acetonitrile

70

60 ~

~ ~

50 .~

~ ., "ii

40 Vl

30

Reaction conditions: Catalyst-CCoCr-2. Temperature-70°C. Time-8 h. Catalyst weight-O.lg. Solvent volume-10mI

Figure: 4.2.5. Effect of solvent on styrene oxidation

4.2.3. Styrene Oxidation over the Prepared Catalysts

The oxidation of styrene was carried out over all the prepared catalysts under

the selected reaction conditions (table 4.2.1). Improved selectivity to benzaldehyde

was the major concern.

Table 4.2.1: Optimized reaction conditions for styrene oxidation

Reaction Parameters

Temperature

Time

Styrene: TBHP ratio

Catalyst weight

Solvent

116

Selected condition

70°C

8h

1:2

0.1 g

10 ml Acetonitrile


Table 4.2.2 shows the activity for styrene oxidation over the five series of

chromite spinel catalysts.

Table 4.2.2: Catalytic activity of spinels in styrene oxidation

Catalyst Styrene Benzaldehyde (%)

conversion (wt %)

CCr 49.5 76.7

CFCr-l 50.7 62.9

CFCr-2 66.9 70.9

CFCr-3 50.8 74.1

CF 46.4 75.6

CMCr-l 48.7 59.3

CMCr-2 48.9 65.0

CMCr-3 30.9 61.5

MCr 28.4 54.6

CCoCr-l 61.1 70.4

CCoCr-2 68.9 67.9

CCoCr-3 47.8 69.9

CoCr 29.5 64.1

CNCr-1 58.1 72.4

CNCr-2 65.4 71.5

CNCr-3 52.4 74.2

NCr 16.2 62.0

CZCr-l 44.6 65.6

CZCr-2 62.4 64.6

CZCr'-3 52.5 69.6

ZCr 31.6 72.1

117

Chapter 4

4.2.4. Regeneration and Stability

The stability of the catalysts was tested by recycling experiments with

regenerated catalysts. They were carried out as follows. After 8 h reaction, the catalyst

was recovered by hot filtration, washed several times with acetone, dried at 80°C

overnight and calcined for 8 h at 650°C. The recovered catalysts were reused for

styrene oxidation under the same reaction conditions. The result obtained with

regenerated catalysts is presented in table 4.2.3.

Table 4.2.3. Activity of regenerated catalysts

Catalyst Cycle Styrene conversion Benzaldehyde

(wt %) (%)

1 62.4 64.6

CZCr-2 2 64.2 66.6

3 63.8 68.5

4 61.1 67.5

There was no significant change in styrene conversion and benzaldehyde

selectivity during all cycles. The catalyst demonstrated good reusability and

regenerability .

4.2.5. Discussions

Styrene oxidation was carried out over all the catalysts using TBHP as

oxidant. Moderate styrene conversion and good selectivity to benzaldehyde were

obtained in all the catalysts. Copper chromite showed about 50% styrene conversion.

Iron substitution had an enhanced effect on the activity of copper chromite. Among

the iron systems, CFCr-2 showed maximum conversion of 77% and had a

118


benzaldehyde selectivity of 71 %. Spinel solid solutions were more active than simple

spinels. Iron and chromium on the octahedral site of the spinel had improved activity

since the catalytic activity of spinels mainly depends on the metal ion on the

octahedral position. Copper ferrite was least active among this series of spinel

catalysts.

Manganese substitution led to a decrease in activity of copper chromite spine!.

All the manganese-substituted spinels were less active than copper chromite and

manganese chromite was the least active catalyst among that series. CMCr-2 showed

maximum conversion of about 50%. Among cobalt substituted copper chromites,

cobalt substitution first increased the catalytic activity and at higher composition of

cobalt. the activity decreased. In this case too. the activity of cobalt chromite was low

compared to the solid solutions.

Nickel substitution had also improved the activity of copper chromite towards

styrene oxidation. Among nickel-substituted series, CNCr-2 with equal ratio of nickel

and copper in the tetrahedral position exhibited maximum activity towards styrene

oxidation. Nickel chromite was the least active catalyst among all the systems studied.

CZCr-2 was found to be the most active in the zinc substituted copper chromite

spinels. It gave a styrene conversion of 62.4% with 64.6% selectivity to benzaldehyde.

From all the above observations. it was concluded that copper chromite and

transition metal substituted copper chromites were active in the oxidation of styrene

with TB HP and simple chromites were less active than the spinel solid solutions.


Two reaction pathways are involved in the oxidation of styrene. They are the

oxidative double bond cleavage on styrene to benzaldehyde and epoxidation of

styrene to styrene oxide and further isomerisation to phenyl acetaldehyde. A proposed

119

Chapter 4

mechanism for the oxidation of styrene to benzaldehyde was described in scheme

4.2.2. A radical chain reaction (one electron transferred), leading to benzaldehyde, in

which the TBHP molecularity is two is proposed.

2 C (CH3).00H ----.c __ C (CH3) 3 O· + C (CH3) 3 00' + H20

OOC(CH3)3

H

Styrene

j 00·

H

Benzaldehyde

Scheme 4.2.2: Proposed mechanism for oxidation of styrene in the presence of TBHP

4.2.7. Conclusions

The summary of the results of styrene oxidation reaction is given below .

../ Copper chromite and transition metal substituted copper chromite catalysts

effectively catalyzed styrene oxidation with TBHP.

120


./ The reaction gave benzaldehyde as the major product formed by the oxidative

cleavage of C=C of styrene .

./ Reaction parameters such as time. temperature. styrene: TBHP ratio. catalyst

weight and effect of solvents were studied in detail and reaction conditions

were optimized .

./ Regeneration and stability of the catalysts were also studied and found that

the catalyst was stable up to the four cycles studied .

./ A possible radical chain mechanism involving single electron transfer was

proposed for the formation of benzaldehyde from styrene.

************

121

Chapter 4

SECTION: C

4.3. OXIDATION OF CYCLOHEXANE

4.3.1. Introduction

The selective oxidation of cyc10hexane is one of the most challenging and

promising subjects from synthetic and industrial point of view l.2

• because this process

produces an important KI A oil (a mixture of cyclohexanone and cyclohexanol)

intermediate in the petroleum industrial chemistry. Such oil can be used for the

production of adipic acid and c;- caprolactum. which are key materials for

manufacturing nylon-6.6 and nylon-6 respectivell. More than 106 tonnes of

cyclohexanone and cyc1ohexanol are produced world wide per annum4• Modern

industrial methods usually require high pressure and temperature when using soluble

cobalt as catalyst. which has led to the realization of high selectivity (about 80 %) for

the sum of cyclohexanone and cyclohexanol only at a low conversion (1-4 mol %).

since the products. cyclohexanone and cyclohexanol, are substantially more reactive

than the cyclohexane reactant. Thus, it is difficult to receive high conversion and

selectivity simultaneously under mild conditions. Commonly used oxidants are

molecular oxygen, hydrogen peroxide and alkyl hydro peroxide. The reaction

pathways involved in the cyclohexane oxidation is shown in scheme 4.3.1.

o [0] 60H

60

0 ... + + + CHiCH2)4CHO

n-hexanal Cyc1ohexanol Cyc1ohexanone Cyc10hexene Cyclohexane

Scheme: 4.3.1. Reaction scheme of cyclohexane oxidation

122


Many efforts have been made to develop new catalysts to oxidize cyclohexane

under mild conditions with high selectivity for the target products using different

oxidizing agents5.6. A noteworthy development in this regard was the results by

Thomas et al. in that their catalyst. FeAIPO-31. allowed for a clean. solvent free one

step process. albeit with a Significant co-production of adipic acid7• Zhou et al.

prepared nanocrystals of C030 4 and found it as an effective catalyst in cyclohexane

oxidation to K/A oil with molecular oxygen as oxidant8. A better conversion of 7.6%

conversion and 89.1 % selectivity towards desired products was obtained for a reaction

time of six hour and they proposed a free radical mechanism for cyclohexane

oxidation. Redox metals such as Ti. Co. Fe and Cr were incorporated into the

framework of TUD-l by Maschmeyer and co-workers9 and cyclohexane oxidation

was studied over these catalysts. A conversion close to 3% was achieved with 85%

selectivity towards the desired products over Fe and Ti-TUD-I. Bellifa et al. prepared

20 wt% VzOs-Ti02 mixed oxides by sol-gel route and studied cyclohexane oxidation

in the presence of acetic acid as solvent and acetone as initiator. The catalyst showed

an appropriate 8% conversion with 76% selectivity towards KI A oil 10.

Metal containing mesoporous materials such as Ti-MCM-41. Cr-MCM-41. V

MCM-41. Bi-MCM-41 and V-MCM-48 were applied to catalyze the oxidation of

cyclohexanell.18

. Metal complexes and metal containing zeolites were also used as

catalysts for this oxidation reaction19Z6. Elements such as V. Sn. Cr. Zr and W could

have been immobilized in crystalline or amorphous silica matrices27-34

. The activity of

these materials in liqUid phase oxidation had generally been correlated with the redox

properties of these elements.


123

Chapter 4

The influence of different reaction parameters was analyzed in order to

maximize the product yield and selectivity. Effect of reaction conditions for

cycIohexane oxidation with TBHP was initially assayed in non-optimized conditions

with CNCr-2 as the catalyst.

4.3.2.1. Effect of Time

The influence of reaction time an cycIohexane conversion and product

selectivity is shown in figure 4.3.1.

~ Cyc10hexane --.- Cyclohexanone --v- Cyc1ohexanol

70 --------------'1 50

3 8 10 12 24

Tirne(h)

Reaction conditions: Catalyst-CNCr-2. Temperature-70°C. Cyclohexane: TBHP =1:2. Catalyst weightO.lg. Solvent -1 OmI Acetonitrile

Figure 4.3.1: Effect of time on cyclohexane oxidation

CycIohexane conversion increased with time. A high conversion of about

59% was achieved at 24 hours. The ketone selectivity was found to be increased with

time correspondingly a decrease in selectivity of the alcohol was observed. The

decrease in selectivity of cyclohexanol could be explained as follows. The catalyst

was active and well promoted reaction with cyclohexanol, possibly the oxidation to

form cyclohexanone. Also cyc1ohexanol was dehydrated to form cyclohexene.

124


Moderate cyc10hexane conversion and KJ A oil selectivity was obtained after 10 h

reaction and this time was selected for further investigations.


The dependence of reaction temperature on cyclohexane was studied by

carrying out this reaction at various reaction temperatures from 60°C to 90°C and the

results are given in figure 4.3.2.

OOr-------------------------~~

~" . ~ 40

20 ~---~ !O+------~------~----~---j.lO

70 00

TeJqlerature ("0

Reaction conditions: Catalyst-CNCr-2, Time-10 h. Cyclohexane: TB HP = 1: 2. Catalyst weight -0.1 g. Solvent -1 Oml Acetoni trile

Figure 4.3.2: Effect of temperature on cyclohexane oxidation

Cyclohexane conversion reached about 24% when temperature reached 70°C

with 71% selectivity to KJ A oil. The percentage conversion was 16.6% when the

temperature was 60°C. An increase in temperature increased the conversion rate up to

70°C and then decreased. From the results it could be concluded that the oxidation of

cyclohexane proceeded with high activity and selectivity under gentle reaction

temperature. The decomposition of TBHP to alcohol and oxygen will take place at

125

Chapter 4

higher temperatures and cannot be consumed during the reaction. An optimum

temperature of 7aoc was selected for further studies.

4.3.2.3. Effect of Mole Ratio

The effect of cyclohexane: TBHP ratio in the oxidation of cyclohexane is

presented in figure 4.3.3 .

....... Cydohexane -+- Cyclohexanone --v-- Cyclohexanol

60 -:-, --------------,60

,--. 50

~ ! 40 l': Q

'E 30

§ U 20

I 50

20

10 +--------..,----~-----+ 10

1:01 1:02 1:03 1:05

Cyclohexane:TBHP

Reaction conditions: Catalyst-CNCr-2, Time-l a h. Temperature-7aoC, Catalyst weight -0.1 g. Solvent-10ml Acetonitrile

Figure 4.3.3: Effect of Cyclohexane: TBHP ratio on cyclohexane oxidation

Cyclohexane conversion increased with increase in volume of TBHP.

Meanwhile. the selectivity of KI A oil decreased and the selectivity of cyclohexene.

the dehydrated product of cyclohexanol was increased. About 50% cyclohexene was

obtained at a cyclohexane: TBHP ratio of 1 :5. Moderate conversion and selectivity to

the desired products were obtained with a mole ratio of 1:2 and this was taken as the

optimum ratio for further studies.

4.3.2.4. Effect of Catalyst weight

126


In heterogeneous catalysis, the amount of the catalyst plays an important role

in determining the rate of the reaction. To study this, the catalyst weight was varied by

taking different amount of CNCr-2 catalyst. Figure 4.3.4 shows the influence of

catalyst weight on the cyclohexane oxidation reaction.

___ Cydohexane ---.- Cydohexanone --<>--- Cyclohexanol

60 ----------------, 60

* * • so

* 50 -

~ !. 40 -= .. . ~

40 -;

t 30· .. i = , o I U 20 ~

30

i - 20

i ./

./ 10-~-L-~----~------~ 10

0.05 0.1 0.15 0.2

Amount oC catalyst(g)

~ '"' ~ "i> Vl

Reaction conditions: Catalyst-CNCr-2, Temperature-70°C. Time-lO h, Cyclohexane: TBHP-l: 2. Solvent-10 ml Acetonitrile

Figure 4.3.4: Effect of catalyst weight on cyclohexane oxidation

An initial sharp increase in percentage conversion was observed when the

catalyst amount was increased to 0.1 g. After that percentage conversion reduced and

then remained almost constant. A gradual change in the product selectivity was also

noticed with change in catalyst weight. An optimum catalyst weight of O.lg was

selected for the present reaction. conSidering the percentage conversion and product

selectivity.

4.3.2.5. Effect of Solvent

127

Chapter 4

Catalytic activity depends largely on the nature of the solvent used. So it is

necessary to find out an ideal solvent for the oxidation of cyclohexane with TBHP as

the oxidant. The influence of solvents like benzene. methanol. dichlorobenzene and

acetonitrile on the oxidation is presented in figure 4.3.5.

____ Cyclohexane -.- Cyclohexanone -<>- Cyc1ohexanol

Solvent A-Benzene. B-Methanol. C-Dichlorobenzene.

D- Acetonitrile

Reaction conditions: Catalyst-CNCr-2. Time-IO h. Temperature-70 D C. Cyclohexane: TBHP-I :2. Catalyst weight-O.lg. Solvent volume-IOml

Figure 4.3.5: Effect of solvent on cyclohexane oxidation

No considerable conversion was obtained on solvents like benzene.

dichlorobenzene and methanol. Acetonitrile was selected as the solvent for this

reaction as moderate conversion and selectivity to desired products obtained with this

solvent.

4.3.3 Cyc10hexane Oxidation over the Prepared Catalysts

The above observations revealed that reaction parameters play an important

role in determining the oxidation rate and product selectivity in the liquid-phase

oxidation of cyclohexane using TBHP as oxidant. The oxidation of cyclohexane

reaction was carried out over all the prepared catalysts under the selected reaction

128


conditions (table 4.3.1) in order to produce the desired product K/ A oil more

selectively.

Table 4.3.1: Optimized reaction conditions for cyclohexane oxidation

Reaction Parameters

Temperature

Time

Cyclohexane: TBHP ratio

Catalyst weight

Solvent

Selected condition

70°C

10 h

1:2

0.1 g

Acetonitrile 10 ml

Table 4.3.2 shows the activity for cyclohexane oxidation over the five series

of chromite spinel catalysts.

Table 4.3.2: Catalytic activity of spinels in cyclohexane oxidation

Catalyst Cyclohexane Product selectivity (%)

conversion Cyclohexanol Cyclohexanone Cyc10hexene

(wt%)

CCr 12.2 28.9 44.7 26.4

CFCr-1 14.2 34.5 43.4 22.1

CFCr-2 17.4 29.1 52.9 17.9

CFCr-3 19.0 24.9 47.6 27.5

CF 15.9 27.7 44.5 27.8

CMCr-1 18.5 23.9 47.5 28.6

CMCr-2 21.2 25.9 50.6 23.5

CMCr-3 18.6 26.4 44.4 29.2

MCr 16.5 28.5 44.5 27.0

CCoCr-1 22.1 19.2 45.5 35.3

CCoCr-2 20.8 21.2 49.9 28.9

129

Chapter 4

CCoCr-3 20.9 20.5 45.5 34.0

CoCr 21.8 15.6 56.5 27.9

CNCr-1 15.7 26.9 46.0 27.1

CNCr-2 19.7 26.7 48.6 24.7

CNCr-3 14.9 25.1 48.2 26.7

NCr 5.4 32.9 41.4 25.6

CZCr-1 14.4 25.6 50.9 23.5

CZCr-2 16.8 23.6 52.1 24.3

CZCr-3 16.6 23.9 49.4 26.7

ZCr 23.3 12.3 64.4 23.3


To study the stability of the catalysts, recycling experiments were carried out

with regenerated catalysts. The recycling experiments were carried out as follows.

After 10 h reaction, the catalyst was recovered by hot filtration, washed several times

with acetone, dried at 80°C overnight and calcined for 8 h at 650°C. The recovered

catalysts were reused for cyclohexane oxidation under the same reaction conditions.

The result obtained with regenerated catalysts is presented in table 4.3.3.


Catalyst Cycle Cyclohexane Product selectivity (%)

conversion Cyclohexanol Cyclohexanone CycIohexene

(wt %)

1 16.6 23.9 49.4 26.7

CZCr-3 2 18.8 22.9 45.3 31.8

3 17.1 20.4 43.8 35.8

4 18.4 20.1 47.5 32.4

130


It was observed that the activity of CZCr-3 did not decreased during the four

runs. The catalytic performance remained stable. as proved by the similar conversion

of cyclohexane and product selectivity for second third and fourth run. This showed

that CZCr-3 was a highly active. selective and stable heterogeneous catalyst for the

oxidation of cyclohexane.

4.3.5. Discussions

The liquid-phase oxidation of cyclohexane was performed over all the

catalysts prepared at 70°C using TBHP as oxidant. The products obtained were

cyclohexanol. cyclohexanone (K/A oil). cyclohexene and trace amount of n-hexanal.

The percentage of n-hexanal was very low and was neglected. In all cases.

cyclohexanone selectivity was higher than that of other two products. The reason for

the higher selectivity of cyclohexanone was that. in the reaction medium.

cyclohexanol formed had a tendency to oxidize to cyclohexanone. Cyclohexanol also

underwent dehydration resulting in the formation of cyclohexene.

Cyclohexane oxidation over copper chromite catalyst resulted in 12.2%

conversion with about 73% selectivity to KI A oil. Iron substitution had an enhanced

activity towards cyclohexane oxidation and CFCr-3 gave a maximum conversion of

19%. Moderate cyclohexane conversion and good selectivity towards KlA oil was

observed in all this catalyst series. Among manganese substituted copper chromites.

CMCr-2 gave a maximum conversion of 21.2% with 76% selectivity to alcohol and

ketone.

Cobalt substitution improved the catalytic activity of copper chromite towards

cyclohexane oxidation. Cyclohexane conversion of more than 20% and good

selectivity to KlA oil was observed in all cases. Though nickel substituted spinels

exhibited higher conversion than copper chromite. these systems gave only below

20% conversion to cyclohexane. Among this series of spinels. CNCr-2 gave the

131

Chapter 4

maximum conversion of 19.7% and least activity by nickel chromite, only 5.4%

conversion. Zinc substituted copper chromites were found to be the least active

catalysts towards cyclohexane oxidation. Zinc chromite exhibited maximum activity

with 23.3% conversion of cyclohexane.


A proposed reaction mechanism for spine I catalyzed cyclohexane oxidation is

depicted in scheme 4.3.2. TBHP is decomposed on the catalyst surface forming t

butoxy radicals with oxidized catalyst. These radicals abstract hydrogen from

cyclohexane forming cyclohexyl radicals, which reacts with molecular oxygen from

air. The cyclohexyl peroxy radicals thus formed can suffer a bimolecular Russell

termination35 to form cyclohexanone and cyclohexanol or abstract a hydrogen from

cyclohexane to form cyclohexyl hydroperoxide. Hydroxy radical abstraction from

cyclohexyl hydroperoxide by the catalysts forms cyclohexyloxy radicals which are in

equilibrium with the open chain isomer, thus forming n-hexanal. The cyclohexyl

radicals may suffer dehydrogenation to cyclohexene by reduction of the catalyst

formed by the decomposition of TBHP, thus regenerating the active catalyst.

132


7 00H __ c_a_tal....:...y_st_. 7 0 .

7 0. + 0 --O· + 7 0H

0·+ O2 ----. 0-00 . • o-OOH +

Russell Termination 0- +

0 j

+ 00· H6

o· j

o + H

o

u~- CH,ICH),CHC

Scheme 4.3.2: Proposed mechanism for oxidation of cyclohexane

4.3.7. Conclusions

The important conclusions of the cyclohexane oxidation reaction are

presented below .

• :. Cyclohexane was effectively oxidized by copper chromite and

transition metal substituted copper chromite spinel catalysts in the

presence of TBHP as oxidant.

.:. The major products obtained are cyclohexanoL cyclohexanone and

cyclohexene. Trace amount of n-hexanal was also detected.

133

Lnapcer.q

.:. Reaction conditions such as time. temperature. cyclohexane: TBHP

ratio. catalyst weight and solvents were optimized in order to

maximize the conversion and selectivity of products .

• :. Reusability study of the catalysts were carried out and found that the

catalysts are stable up to four cycles of the reaction .

• :. A possible reaction mechanism involved in this oxidation was

suggested.

************

134


SECTION: D

4.4. OXIDATION OF ETHYLBENZENE

4.4.1. Introduction

Effective utilization of ethylbenzene. available in the xylene stream of the

petrochemical industry. for more value- added products is an interesting proposition.

Oxidation of ethylbenzene is of much importance for the production of the aromatic

ketone. acetophenone. one of the key products in the industries. It is used as a

component of perfumes and as an intermediate for the manufacture of

pharmaceuticals. resins. alcohols and tear gas (chloroacetophenone). The oxidation

pathways of ethylbenzene are presented in scheme 4.4.1.

0° ~I ~ Benzaldehyde

t CH CH3 /' 3

HO- H =0

Acetophenone

~ ~ CHpH 6CHO

Ethylbenzene '" 6' ~ I --- // .. / I ~ ~

2-Phenyl ethanol Phenyl acetaldehyde

Scheme 4.4.1: Reaction scheme of ethylbenzene oxidation

135

Chapter 4

Cobalt containing hexagonal mesoporous molecular sieves prepared by direct

hydrothermal and post-synthesis method was found to be very effective in the

oxidation of ethylbenzene to acetophenone l. The results showed that solvent had a

negative impact over the performance of the cobalt-containing catalyst, which arouse

from the blocking of active sites by the solvent molecules. Recently, Jana et al.2

reported NiAI hydrota1cite to be an environmentally friendly solid catalyst for the

liquid-phase selective oxidation of ethylbenzene to acetophenone with molecular

oxygen. They have proposed a free radical mechanism in this case.

Most of the oxidations of alkyl aromatics were carried out in the slurry phase

using sacrificial oxidants such as HZ0 2 and TBHP. Mal and Ramaswamy have used

Ti, V and Sn containing silicates and obtained 62% product distribution of

acetophenone in the low temperature region of 30-50°C under liquid phase reactions3.

Vetrivel and Pandurangan used Mn-MCM-41 with various Si/Mn ratios and obtained

10-43% selectivity to acetophenone in the temperature range of 60-80 D C with tert

butyl hydroperoxide as an oxidant4• Srinivas and workers have studied the catalytic

effect of oxo-Mn-triazacyclononane complexes in the liquid phase oxidation of

ethylbenzene5. The studies revealed that nuclearity and type of oxo-Mn speciation

influence the catalytic activity. I-phenylethanol and acetophenone were the benzylic

oxidation products obtained along with a small amount of ortho- and para- ring

hydroxylated compounds. Copper tri- and tetraaza macro cyclic complexes

encapsulated in zeolite-Y exhibited good catalytic performance in the oxidation of

ethylbenzene using TBHP as oxidant6• Acetophenone was the major product with

small amounts of 0- and p-hydroxyacetophenones indicating that C-H bond activation

takes place both at benzylic and aromatic ring carbon atoms. Soluble acetylacetonate

nickel (II) complexes were used for ethylbenzene oxidation with quaternary

ammonium salts and macro cyclic polyethers7. Ethylbenzene oxidation with TBHP by

136


polynuclear Mn Schiff base complexes produced acetophenone and I-phenyl ethanol

with small amounts of peroxy compoundsB• Selective oxidation of ethylbenzene with

air produced I-phenyl ethanol and acetophenone over dimeric metalloporphyrins9.

According to Singh et al.lO. the redox behavior of MeAPO-ll had a potential

influence on the catalytic activity during the oxidation of ethylbenzene with TBHP.

Reddy and Varmall prepared Alz03 supported VZ0 5 catalyst and the liquid phase

oxidation of ethylbenzene to acetophenone was employed as a chemical probe

reaction to examine the catalytic activity. Toribio et alY reported the liquid-phase

ethylbenzene oxidation to hydroperoxide with barium catalysts. Along with

ethylbenzene hydroperoxide as the major product. small amounts of acetophenone and

I-phenyl ethanol were obtained indicating that C-H bond activation takes place only

at the alkyl chain.

The oxidation of organic substrates using HzOz as oxidant has been well

documented13-15

. According to Xavier et al. I6 Y- Zeolite encapsulated Co (II). Ni (11)

and eu (II) complexes gave acetophenone as the only partial oxidation product during

ethylbenzene oxidation with HZ0 2• The catalytic activity was attributed to the

geometry of encapsulated complexes. Titanosilicates mainly catalyze ring

hydroxylation of arenes with HZ0 2: whereas vanadium and chromium substituted

zeolites and an aluminophosphate molecular sieve have been known to favor side

chain oxidation selectivelyl7. Rebelo et al. 18 studied the oxidation of alkyl aromatics

with HZ0 2 over Mn (Ill) porphyrins in the presence of ammonium acetate as co

catalyst. The catalysts produced acetophenone as the major product with 1-

phenylethanol. 2-ethyl-l, 4-benzoquinone and styrene. Oxidation took place mainly in

the benzylic positions with these catalysts. Products arising from further oxidation of

acetophenone were not detected. The oxidation of alkyl benzenes with HZ0 2 over Cu

(II) complexes took place selectively at the benzylic C-H bond without any oxidation

in the remaining C-H bonds l9. During liquid-phase oxidation of ethylbenzene with

137

Chapter 4

molecular oxygen over quaternary ammonium compounds ethylbenzene

hydro peroxide was the main product2Q,21,

A broad variety of catalytic systems have been described in the literature for

vapour-phase oxidation of ethylbenzene with air. Vetrivel and Pandurangan have

reported the catalytic behavior of Mn-MCM-41 mesoporous molecular sieves in the

vapour-phase oxidation of ethylbenzene22, Acetophenone was obtained selectively

than benzaldehyde and styrene, They have also reported this reaction over

mesoporous MCM-41 and AI-MCM-41 23, The catalytic oxidation of ethylbenzene to

ethylbenzene hydroperoxide with air in liquid phase using Ni (II) complexes resulted

mainly reaction byproducts as acetophenone and phenof4, )..I.-oxo dimeric

metalloporphyrins25, bis (acetylacetonate) nickel (II) and tetra-n-butyl ammonium

tetrafluoro borate26 were also employed as catalyst for this reaction.


The influence of different reaction parameters was analyzed in order to

maximize the product yield and selectivity since reaction conditions have a critical

role in a chemical reaction. Effect of reaction conditions for ethylbenzene oxidation

with TBHP was initially assayed in non-optimized conditions with CNCr-2 as the

catalyst.

4.4.2.1. Effect of time

The effect of time on ethyl benzene oxidation reaction is depicted in figure

4.4.1. Ethylbenzene conversion was nearly steady up to 6 h and then increased

significantly. Similarly, acetophenone selectivity, which was nearly steady, improved

suddenly and then remained almost constant. Even though acetophenone was the

major product of the reaction, 1-phenylethanol and a small amount of benzaldehyde

were obtained as side products. Reaction time of 8 h was selected for further studies,

138

70

65

* :! 60

g 55 . "E I .. [; 50 o u 45

- __ Ethylbenzene ---fr- Acetophenone

/~ : /

/ /

/ .-


r 70

65 ~

l 60 ~

';:l

~ .. 55 '"

40+-~~----~----~--~----~50

4 8 10

Time(h)

Reaction conditions: -Catalyst-CNCr-2, Temperature-70°C, EB: TBHP ratio- 1 :2, Catalyst weight-O.lg, Solvent -1 Oml Acetonitrile

Figure 4.4.1: Effect of time on ethylbenzene oxidation


Reaction temperature has an important role in conversion rate and product

selectivity. The effect of temperature on ethylbenzene oxidation with TBHP is

presented in figure 4.4. 2.

70 .• Ethylbenzene -6' Acetophenone

'#: 60

·s

,80

! 75

70 ~ ~

65 € 60 ~

~ so /Y/~-'----'''--------.

~ / 8 40 / ___ .__ .

30+--.--~--~----~--·---!~---~----~--+_::J5 i '"

60 70 80 90 100 Temperature('C)

-------

Reaction conditions: - Catalyst-CNCr-2. Time-8 h, EB: TBHP ratio- 1:2, Catalyst weight-0.1g, Solvent-10ml Acetonitrile

Figure 4.4. 2: Effect of temperature on ethylbenzene oxidation

139

Chapter 4

As temperature was raised, the conversion of ethylbenzene increased initially

and remained steady later. Selectivity towards acetophenone was nearly constant up to

70a C, decreased moderately at 80a C and was almost constant later on. A temperature

of 70a C was selected for further studies.

4.4.2.3. Effect of reactant mole ratio

The effect of ethylbenzene: TBHP mole ratio on the oxidation of

ethylbenzene is presented in figure 4.4.3.

[00 ~ Ethylbenzene -A- Acetophenone 70

~ 90 60

1 80

c 70 Cl .;:;; .. .. 60 ;>

c Cl

U 50 30

40 +-----~----~----~----_+20 [:o[ 1:02 1:03 [:05

EB:TBHP

Reaction conditions: - Catalyst-CNCr-2, Time-8 h. Temperature-70a C. Catalyst weight-O.lg. Solvent-10ml Acetonitrile

Figure 4.4.3: Effect of mole ratio on ethylbenzene oxidation

A sharp increase in conversion was observed on increasing ethylbenzene:

TBHP ratio up to 1:3 after which the conversion remained constant. Selectivity

towards acetophenone showed the reverse trend. A higher concentration of TB HP

increased the rate of conversion of I-phenylethanol to benzaldehyde and hence a

decrease in selectivity of ketone was observed. A mole ratio of 1: 2 was selected for

the study.

140


4.4.2.4. Effect of catalyst weight

Figure 4.4.4 depicts the effect of catalyst weight on ethylbenzene oxidation.

60

~ 58

!. 56 ] ::: 54 ~ co U 52

___ Ethylbenzene ~ Acetophenone 80

50+-----~----~----_.-----+40

0.05 O.l 0.l5 0.2

Amount of catalyst(g)

Reaction conditions: - Catalyst-CNCr-2, Time-8 h, Temperature-70°C, EB: TBHP- 1:2, Solvent-IO ml Acetonitrile

Figure 4.4.4: Effect of catalyst weight on ethyl benzene oxidation

Ethylbenzene conversion increased gradually with increasing catalyst amount.

Change in acetonitrile selectivity was similar but with a more significant increase. A

catalyst weight of 0.1 g was selected for further studies.

4.4.2.5. Effect of solvent

Solvents play a decisive role in liquid phase reactions in influencing both the

conversion as well as product selectivity. Effect of solvents on ethylbenzene oxidation

was studied and the results obtained are given in figure 4.4.5. The reaction was also

carried out in the absence of solvent.

Maximum ethylbenzene conversion was observed when no solvent was used.

Addition of solvent decreased the conversion. The decrease in conversion was

attributed to the blocking of active sites by solvent molecules 1. Even though

141

Chapter 4

maximum conversion was obtained without solvents, the reaction was carried out with

10 ml acetonitrile solvent.

60 90

50

* ..; 40 -!.

85

80 ~ >.

§ 30 .:;; 75 ~ .. ~

20 > I::i 0

'" 70 ~ Vl

<.) 10 55

0 60 A B SdlVent D E

A-Without solvent, B-Acetonitrile, C-Chlorobenzene. D-Benzene, E- Dichloromethane

Reaction conditions: - Catalyst-CNCr-2, Time-8 h, Temperature-70 c e, EB: TBHP- 1:2.Catalyst weightO.lg, Solvent Volume-lOml

Figure 4.4.5: Effect of solvent on ethylbenzene oxidation

4.4.3. Ethylbenzene Oxidation over the Prepared Catalysts

The oxidation of ethylbenzene reaction was carried out over all the prepared

catalysts under the selected reaction conditions (table 4.4.1) with an aim to produce

acetophenone more selectively.

Table 4.4.1: Optimized reaction conditions for ethylbenzene oxidation

Reaction Parameters

Temperature

Time

Ethylbenzene: TBHP ratio

Catalyst weight

Solvent

142

Selected condition

70°C

Bh

1:2

0.1 g

Acetonitrile 10 ml


Table 4.4.2 shows the activity for ethylbenzene oxidation over the five series

of chromite spinel catalysts.

Table 4.4.2: Catalytic activity of spinels in ethylbenzene oxidation

Catalyst Ethylbenzene Product selectivity (%)

conversion Acetophenone I-phenylethanol Others

(wt %)

CCr 32.9 13.9 83.4 2.7

CFCr-1 43.3 39.1 52.6 8.3

CFCr-2 57.6 55.2 40.8 4.0

CFCr-3 60.2 55.6 26.3 18.1

CF 52.3 54.0 35.6 10.4

CMCr-1 52.7 53.8 35.7 10.5

CMCr-2 51.6 56.7 34.5 8.8

CMCr-3 52.9 62.6 27.8 9.6

MCr 37.7 52.4 35.1 12.5

CCoCr-l 50.4 65.0 29.5 5.5

CCoCr-2 52.9 65.9 28.3 5.8

CCoCr-3 55.3 61.8 36.3 2.9

CoCr 52.1 69.9 26.1 4.0

CNCr-l 44.7 51.9 46.4 1.7

CNCr-2 56.1 68.7 28.1 3.2

CNCr-3 55.5 55.6 39.6 4.8

NCr 20.2 59.1 19.4 21.5

CZCr-1 51.5 70.1 24.8 5.1

CZCr-2 53.7 57.5 33.5 9.0

CZCr-3 53.9 65.9 30.2 3.9

ZCr 49.4 66.4 30.0 3.6

143

Chapter 4


To study the stability of the catalysts. recycling experiments were carried out

with regenerated catalysts as follows. After 8 h reaction. the catalyst was recovered by

hot filtration. washed several times with acetone. dried at 80°C overnight and calcined

for 8 h at 650°C. The recovered catalysts were reused for ethylbenzene oxidation

under the same reaction conditions. The result obtained with regenerated catalysts is

presented in table 4.4.3.


Catalyst Cycle Ethylbenzene Product selectivity (%)

conversion Acetophenone 1-phenylethanol Others (wt%)

1 53.9 65.9 30.2 3.9

CZCr-3 2 58.6 72.4 13.9 13.7

3 54.9 72.8 12.7 14.5

4 51.5 78.0 14.9 7.1

Ethylbenzene conversion remained almost constant over four catalytic runs.

Acetophenone selectivity increased after first cycle and then remained nearly steady.

while I-phenylethanol selectivity decreased. It was concluded that the catalyst was

stable up to four cycles.

4.4.5. Discussions

Ethylbenzene oxidation over all the prepared catalysts was carried out at 70°C

in liquid phase using tertiary butyl hydro peroxide as the oxidizing agent. The major

products obtained were I-phenylethanol and acetophenone along with minor products

such as phenyl acetaldehyde and benzaldehyde. These two minor products together

were included in the others category. Ethylbenzene first reacted with TBHP to

144


produce I-phenylethanol, which was then oxidized to acetophenone. the major

product.

Among the catalytic systems studied. copper chromite showed least

selectivity to acetophenone. The conversion of I-phenylethanol to acetophenone was

very low in this case. Other products obtained are very less. below 3%. Iron

substitution had a remarkable influence on the activity of copper chromite. About 60%

conversion and 56% selectivity towards acetophenone was observed on CFCr-3

catalyst.

Manganese substituted copper chromite spine Is were very active towards

ethylbenzene oxidation. Above 50% ethylbenzene conversion and more than 50%

selectivity towards acetophenone was observed in all the substituted catalysts.

Meanwhile, manganese chromite showed the least activity among those catalytic

systems.

Maximum selectivity towards acetophenone was observed on cobalt

substituted copper chromite spinels. Above 60% selectivity to acetophenone was

observed in all the systems along with more than 50% ethylbenzene conversion.

Nickel substitution also had an enhanced effiCiency in the oxidation of ethylbenzene.

Among all the spinel systems studied. nickel chromite was least active. The selectivity

towards I-phenylethanol was also less and that of other products was more. Zinc

substituted copper chromite spinels were also very active in ethylbenzene oxidation.


A plausible mechanism for the oxidation of ethylbenzene is presented in

scheme 4.4.2. TBHP was activated by co-ordinating with metal oxide. The activated

distant oxygen of co-ordinated TBHP reacted with ethylbenzene to yield the products.

I-phenylethanol from ethylbenzene was produced by insertion of oxygen between

145

Chapter 4

carbon hydrogen bond of the methylene group. Abstraction of an alcoholic OH

hydrogen and the CH hydrogen by the activated t-butylhydroperoxide oxygen yielded

acetophenone. Similar abstraction of OH hydrogen of I-phenylethanol by the

activated t-butylhydroperoxide yielded benzaldehyde. The methyl group of

ethylbenzene was also be attacked by activated t-butylhydroperoxide to yield 2-

phenylethanol, which was very rapidly oxidized to phenyl acetaldehyde.

CH CH3 I 3 /

H06: :;;-OOH 6: - 1 .1 o MO 0

MO ~ 700H

,...CHO

o

MO ~ :;;-OOH

Scheme: 4.4.2: Proposed mechanism for oxidation of ethylbenzene

4.4.7. Conclusions

The following conclusions can be drawn from the present study.

146


.:. The oxidation of ethylbenzene with t-butylhydroperoxide over copper

chromite and transition metal substituted copper chromites catalysts gave 1-

phenylethanol and acetophenone as the major products. Trace amounts of

benzaldehyde and phenyl acetaldehyde were also detected .

• :. The influence of reaction variables such as reaction time. temperature of the

reaction. ethylbenzene to TBHP ratio. catalyst weight and solvent were

studied and oxidation reaction was carried out under the optimized conditions .

• :. Regeneration and stability of the catalysts were studied and the results proved

that they were stable up to four reaction cycles .

• :. A plausible mechanism involved in the oxidation of ethylbenzene was also

proposed.

************

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152

oxidation of hydrocarbons abstract -...

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